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Grade Level: 9 – 12
Purpose:
The purpose of this lesson is to introduce sedimentary rocks. Students
will learn what sedimentary rocks are and how they form. It will teach
them how to identify some common examples.
Suggested Goals:
Students will shake a flask of sand and water to see how particles settle.
They will draw a picture showing where the various rocks form and they
will identify some common examples.
Targeted Objectives:
All of Illinois is covered by sedimentary rocks. In most places, they
reach a thickness that could be expressed in miles rather than feet. Ancient
seas dropped untold numbers of particles for hundreds of millions of years
creating the layers that bury the original igneous rocks in 2,000 to 17,000
feet of sedimentary layers. These valuable layers are used to construct
our homes, build our highways, and some were even used to heat our homes.
The Illinois Basin dominates the geology of southern Illinois. There
the layers dip down until they are over three miles thick. The geology
of Illinois cannot be told without discussing sedimentary rocks because
they are what Illinois is made of.
- Students will learn to tell the difference between the major sedimentary
rock varieties.
- Students will learn in what environments different sedimentary rocks
form.
- Students will learn what sedimentary rocks are.
Background:
Most sedimentary rocks are formed when weathering crumbles the parent
rock to such a small size that they can be carried by wind or water. Those
particles suspended in water collide with one another countless times
gradually becoming smaller and more rounded. When water is moving quickly
due to flooding or due to a rapid change in elevation, larger particles
can be carried by the streams and rivers but when the water slows down
the particles begin to settle out. The particles dropped from water and
wind are called sediment. As layers become thicker, the bottom particles
get squeezed closer and closer together under the crushing weight.
Many of the organisms that live in the oceans have shells and skeletons
that are made of calcium carbonate. Their decay releases a natural cement
into the water. The cement glues the particles of rock back together forming
sedimentary rock. People who live in homes with hard water see the amazing
affect of this calcium carbonate glue in their showerheads as the holes
gradually become closed until the spray is reduced to a trickle.
The variety of sedimentary rock often depends on the size of the particles
that make up the rock. Conglomerate, sandstone, and shale form from different
sized particles.
Conglomerate usually forms in places with fast moving water.
Finding conglomerate made of large particles often suggests that when
they were formed there were mountains nearby. Conglomerate means a variety
and that is a fitting name because there are usually large and small particle
intermingled within the rock.
Sandstone forms where the flow is slower. Sand grains bounce
down streams without much difficulty so even slow moving streams can carry
vast amounts of sand.
Shale is made from silt (dirt) in the water. The particles are
so small that they can stay suspended in water for quite a while even
after the river or stream has stopped flowing. The silt gradually settles
to the bottom.
Limestone and dolomite do not form from particles washed from the land.
They usually form on the ocean bottom far out to sea where no particles
from the land ever reach.
Limestone is made from an infinity of microscopic skeletons
that rain down on the sea floor. In some cases the shells of larger organisms
such as brachiopods are found, too. Limestone is almost totally calcium
carbonate.
Dolomite is similar to limestone except that it forms in the
ocean in places that have magnesium dissolved in the water. It often forms
from limestone as the calcium in calcite is partially replaced by magnesium.
Activity: Particles in Sedimentary Rocks
Materials and Preparation:
- Several 500 ml flasks (or larger) filled with 250 ml of sandbox (river)
sand and then filled to 400 ml with water. Tightly seal each with a
cork. A long jump pit is a good place to find the sand.
- Containers to catch any water that might spill from the flasks
- Typing paper
- Colored pencils
- Collections of limestone, sandstone, shale, and conglomerate. If possible,
have enough collections so that there can be groups of two or three.
- Magnifiers
Procedure:
Pass out the flasks of sand created as described above and give each group
a container for spills; in case there is an accident. Have the students
shake the flasks quickly back and forth and then turn them while the water
is still churning so that the cork end is down. Stop shaking. Have the
students observe the sand and answer the following questions.
Discussion:
- Where are the larger particles and why? [Near the bottom because they
could not stay suspended in the water very long when the shaking stopped.]
- Are there just large pieces at the bottom? [No the large pieces are
surrounded by hundreds of grains of sand.]
- As you look higher in the flask what is happening to the size of the
sand grains? [They are getting smaller.]
- Did all of the particles settle out right away? [No the water remained
cloudy and a thin layer of silt is being deposited on top.]
- When a river enters the ocean, the water slows down suddenly just
like when you stopped shaking the flask. Which particles would you expect
to drop closest to the mouth of the river? [The largest ones.]
- If you were to sample the sand near a rivers mouth and then take another
sample father out to sea, what would you expect to find? [Farther out
the sand particles would be smaller.]
- What would settle out even farther that the sand? [Silt (mud)]
Activity: Formation of Particles
Preparation And Procedure:
Draw the following diagram on the board and have your students
copy it onto a piece of typing paper. Have them label and color it for
a homework assignment. As you draw the diagram point out why each particle
forms where it does.
Gravels are close to shore because they are too large to stay
suspended. Sometimes gravel does not occur if the stream flow is too slow.
Sand is the next particle to settle out because it is the next
largest but the sand grains become smaller and smaller the farther they
are from shore.
Mud is next to settle out because the tiny particles of silt
can stay suspended for a very long time, like they did in the flask, after
the water has stopped moving.
The material farthest out forms from shells because no material from
the land can make it far out to sea. This is where limestone forms. The
only things that settle on the bottom far out to sea are those from the
sea itself such as shells and dissolved minerals.
Discussion:
- How would sedimentation be different in an environment where a very
slow moving river enters the ocean? [The first particles to settle out
would be sand or mud.] Where there was a fast current? [The first particles
would be larger.]
- None of the beaches in Florida have gravels. What is the major reason?
[Florida is very flat. The highest point is only 345 feet and most of
the state is much lower. Florida streams are very slow moving so sand
particles are the largest particles that they can carry.]
- What property of sedimentary rocks is used to identify most of them?
[The size of the particles]
- Which sedimentary rock is NOT identified by its particle size? [Limestone;
its identification is based on the chemical that it is made of which
is calcium carbonate.]
Activity: Identification of Sedimentary Rocks
Materials:
- Collections of sedimentary rocks
- Magnifiers
- A drawing from the previous activity for each group
- Student lab sheet (available in the PDF download
version of this lesson)
- Weak hydrochloric acid or vinegar in acid bottles
CAUTION! If you are diluting strong hydrochloric acid be sure to add
the acid to water and NOT water to acid. Have your students wear safety
goggles if they are using hydrochloric acid.
Preparation and Procedure:
Prepare several collections of rocks so that each contains a specimen
of sandstone, limestone, shale, and conglomerate. Make enough so that
you will have enough sets for groups of two or three students.
Hint…Small baskets, boxes, or baggies work well and can be kept
from year to year.
Purchase dilute hydrochloric acid and place it in acid bottles with droppers.
If you are mixing the acid from concentrate, be sure to add the acid slowly
to water and not water to the acid. Wear gloves, long sleeves, and safety
glasses while doing the mixing.
If you are using vinegar, make sure before the lab that the limestone
specimens you are using will bubble when a drop of vinegar is placed on
them. Some do not!
Have your students use the descriptions on their lab sheet to identify
each specimen. Have the students use one of their groups drawings for
this activity. Ask your students to place each variety of rock on the
section of their group’s drawing where it would occur. A piece of
conglomerate will be placed near the shoreline where it appears on the
picture and so forth.
Assessment:
- Correct the pictures showing the environment where each rock variety
would form.
- Give a lab test in which each student identifies each of the four
specimens of sedimentary rocks.
- Have your students answer the following essay questions:
- How are sedimentary rocks different from one another?
- What conditions would produce each variety of sedimentary rock
and where each variety of sedimentary rocks would form?
- If forces were to create mountains in Florida, how would that
affect the beaches there?
- Have your students reproduce the rock drawing from memory.
Extensions:
- Place shale, sandstone in a bag and have students try to identify
them without looking.
- Collect specimens from a stream or other source.
- Have students create sedimentary rock collections.
- Make a list of things made from sedimentary rocks; such things as
plaster, plasterboard, grinding stones, coal, cement, driveways, etc.
- Create a poster similar to the drawing that the students did in class.
Permanently glue specimens to it showing which rock would form in each
place.
Print Resources:
- Geology Underfoot in Illinois by Raymond Wiggers, Mountain
Press Publishing Company, Missoula, Montana. 1997. 800-234-5308
- The Audubon Society Field Guide to Rocks and Minerals. Available
in most bookstores
- The Pocket Guide to Rocks and Minerals by Michael O’Donoghue.
London: Parkgate Books. 1991, 1999. Available in most bookstores
- Peterson Field Guide Rocks and Minerals by Frederick H. Pough.
Boston: Houghton Mifflin Company. 1996. Available in most bookstores
Video Resources:
Web Resources:
Lesson Specifics:
This activity will take about two class periods. It is desirable to have
groups of two or three for the lab portions. Lab tables should be used
so that there is no spillage of the acid.
Illinois State Board of Education Goals and Standards:
- 11.A.4c: Collect, organize and analyze data accurately and
precisely.
- 12.E.4b: Describe how rock sequences and fossil remains are
used to interpret the age and changes in the Earth.
- 12.E.5: Analyze the processes involved in naturally occurring
short-term and long-term Earth events (e.g., floods, ice ages, temperature,
sea-level fluctuations).
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